Based on How i met your mother's 11th Episode of 6th Season
300 years ago sailors stuck at sea would get desperate at sea for female comapionship, it got so bad that manatee out in the water started to look like beautiful womens, mermaids.
Every women no matter how initially repugnant have a mermaid clock, its the time it takes for you to bone her. sure today you see her as a manatee but she aint gonna stay that way.
The mermaid theory it's a thing.
a theory that a woman, no matter how ugly she may seem at first, will eventually look beautiful to you if you see her every day all day.
Based off the theory that sailors used to spend months at sea and started to slowly see manatees turn into mermaids.
Joe: Dude Kelly is so hot!
Tim: Are you nuts?! It's probably just the Mermaid Theory. you have her in every class.